(i) Field of the Invention
The present invention relates to water treatment, and more particularly to an improved water treatment method used to produce drinkable water from impure water.
(ii) Description of the Prior Art
Conventionally, sewage is first pretreated in lagoons to settle out solids and it then passes through first and second filtering tanks or the like which attempt to remove remaining solids. The effluent is then allowed to pass to the disposal outlets which may be rivers, streams or the like. Unfortunately, this effluent still contains a relatively large quantity of impurities, the method is time consuming and relatively large land areas are required for the lagoons.
A co-called "package water treatment plant" is appropriate for use in small community water systems, resort areas or construction and lumber camps located in remote areas. Such package water treatment plant which is easily maintained and operated by unskilled individuals is preferable over larger more complex systems requiring continuous maintenance and supervision. In particular, there is a definite need for package-type water treatment plants which can accommodate flows from about 300 to 6000 gallons per hour and can operate on a continuous daily basis. The size of a package water treatment plant is important, and it is desirable to provide a plant which may be readily moved from one area to another depending upon needs.
It is also well known in the art that the addition of certain chemicals, such as lime and soda ash, to water having undesirable solids dissolved therein has the effect of causing the dissolved solids to precipitate and form a suspension or slurry with the water. It is further known that to add raw water and chemicals to a slurry formed as above results in improved and accelerated coagulation and purification of the water. The solids which percipitate from the raw water tend to deposit by accretion on the particles present in the slurry, forming relatively stable particles which are readily separable from the clear, purified water.
It is also well-known that color, turbidity, organic matter and similar impurities may be removed from water by coagulants, e.g. alum, ferric sulfate or the like. These compounds are acidic and react with the alkalinity in the water or with alkaline compounds, e.g. lime or soda ash, to form voluminous insoluble precipitates (hydrates). The precipitates have a tremendous surface area on which the dissolved or colloidally dispersed impurities are absorbed. The suspended impurities are surrounded by the gelatinous hydrates and become part of the precipitate.
To soften water by this process, lime (calcium hydroxide) is added to precipitate the calcium bicarbonate as calcium carbonate and the magnesium salts as magnesium hydroxide. Soda ash (sodium carbonate) is added to react with the calcium chloride and calcium sulfate originally present in the water as well as that formed by the reaction of lime with magnesium chloride and sulfate. The reaction of sodium carbonate with these salts forms calcium carbonate. Thus the hardness (calcium and magnesium salts) originally present in the water is partially removed as the slightly soluble compounds, calcium carbonate and magnesium hydroxide, precipitate.
Usually a coagulant, e.g. alum, sodium aluminate or ferric sulfate, is added in the treatment to assist in the separation of the turbidity and precipitates formed from the water. If the sterilization and reduction in organic mater are required, chlorine is also used in the treatment. By suitable modification in the chemical treatment, silica reduction can be obtained.
Domestic or sanitary sewage and industrial wastes may be purified by the chemical precipitation process, in which suitable chemicals (e.g. aluminum sulfate, lime, iron chloride, polyelectrolytes or combinations thereof) are added to the sewage and the sewage passed to one or more flocculating tanks, normally equipped with slowly rotating agitators or paddles, in which colloidal solids are formed into particles of size and weight that will settle. The colloidal solids or flocs are then separated from the liquid by being allowed to settle in subsequent settling tanks, whereafter the purified water is collected in a weir structure mounted at the surface of the water, while the sediment, consisting of flocs and sludge, is removed, normally by means of sludge scrapers and/or pumps. Certain industrial wastes or sewage has inherent flocculating tendencies, and thus it is unnecessary with such effluents to add flocculating chemicals since corresponding purifying effects can be obtained solely by agitating the liquid and allowing the resulting floc to settle.
Petersen, Canadian Pat. No. 517,674 issued Oct. 18, 1955 provides method and apparatus for the purification of water. The method for treating water to remove substances therefrom by chemical precipitation comprises contacting the water to be treated with partially clarified water and forming a mixture thereof. Chemicals are added to the mixture so formed and the mixture is passed into a reaction zone under sufficient pressure to cause agitation therein. A sufficient portion of the water displaced from the reaction zone into a primary clarification zone to give a flow rate of from 2 to 14 gallons per minute per square foot of cross-sectional area in the primary clarification zone, the incoming raw water is contacted therewith. The remaining water is displaced from the reaction zone into a secondary clarification zone wherein the precipitated solids settle out slowly. Treated, clarified water is then withdrawn from the secondary clarification zone.
The water treating apparatus includes a treating tank, an inner shell extending downwardly within the tank to a point near the base thereof and communicating at its bottom with the interior of the tank. The inner shell and the tank form an annular passage therebetween. The inner shell has an open-bottomed base portion of greater cross-sectional area than the upper portion thereof. Communicating means are provided between the base portion and the upper portion. A raw water inlet is provided in the upper portion of the inner shell. Means are so arranged as to withdraw water from the upper portion of the inner shell and to introduce such water into the base of the tank at a point immediately below the open bottom of the base portion of the inner shell. Means are provided for introducing chemicals into such water after withdrawal from the inner shell and prior to introduction into the tank. Agitation stilling means are mounted in the annular space between the base portion of the inner shell and the tank. Means are provided for removing solids precipitated from the treated water. Finally, means are provided for removing clarified, treated water from the upper portion of the annular space between the inner shell and the tank.
A package water treatment plant is disclosed in Canadian Pat. No. 692,543 issued Aug. 11, 1964 to C. L. Oldfather. The water treatment plant includes a cylindrical shell disposed with a semicircular settling tank. The cylindrical shell incorporates a plurality of coagulation chambers arranged in continuous communication for eventual discharge of influent impure water into the settling tank. The settling tank discharges water therefrom into a filter which uses sand as the filtering media. After passing through the sand filter, the water is then transported to a storage tank.
Duff et al. Canadian Pat. No. 843,762 issued June 9, 1970 provides a water treating apparatus for softening of water by the cold lime-soda ash process and for the clarification of waters containing suspended solids, color and organic impurities by coagulation with alum or ferric sulfate or other coagulants. Coagulation and softening may be carried out simultaneously in this type of water treating apparatus. Such water treating apparatus has means forming a settling zone, means forming a mixing and recirculation zone and means forming an uptake zone. The uptake zone has a closed bottom member with an opening substantially in the center thereof through which the uptake zone communicates with the mixing and recirculation zone and the settling zone, and the upper portion of the intake zone communicates with the mixing and recirculation zone. The mixing and recirculation zone communicates with the settling zone. Means are provided to supply a chemical treating agent to the uptake zone to form precipitates therein. Means are provided to withdraw effluent from the settling zone. Feed means are provided to pass influent water into the uptake zone, those feed means to pass influent water into the uptake zone being located above the bottom member. These feed means are so constructed and arranged and are the sole means for causing the water in the uptake zone to rotate therein and to be pumped upwardly from the uptake zone and for recirculating precipitates entering the uptake zone from the settling zone through the opening in the bottom member thereof. Those feed means to pass influent water into the uptake zone are connected to a supply of influent water.
Miller, et al Canadian Pat. No. 853,022 issued Oct. 6, 1970 provides a hot process settling tank which includes a generally spherically shell. A partition divided the volume of the spherical shell into an upper reaction compartment and a lower settling compartment. An upwardly diverging baffle is disposed within the spherical shell forming the base surface of the settling compartment and defines the upflow zone of increasing cross-sectional area for gradually decreasing the rate of upflow to permit the formation of sludge blanket in the settling compartment. The baffle and spherical shell together form a generally annular space therebetween which may be used as a liquid storage. A vent is associated with the liquid storage to prevent the entrapment of air therein as the storage space fills with liquid.
Dunhers, Canadian Pat. No. 972,880 issued Aug. 12, 1975 provided a sewage flocculating and sedimentation tank unit for purifying domestic sewage and industrial waste, comprising a flocculating section and a sedimentation section having an outlet and a fluid connection therebetween. The flocculating section and sedimentation section and the connection therebetween are positioned so that the flow of sewage from the flocculating section to the sedimentation section outlet, as seen in plan view, is directed substantially perpendicular to the longitudinal direction of the tank and as seen in vertical section, obliquely upwards. Each of the flocculating section and the sedimentation section is divided into a plurality of aligned compartments by transversely extending guide walls for substantially uniformly distributing the sewage over the tank in the longitudinal direction thereof. Those plurality of aligned flocculating compartments extend the longitudinal length of the tank. The sedimentation compartments confront opposite sides of the flocculating section in fluid communication therewith.
Bultz, Canadian Pat. No. 951,035 issued July 9, 1974, provides a sewage and effluent treating system. The secondary sewage treatment apparatus includes means to add a flocculating chemical to the sewage. Means are provided to agitate the sewage and the chemical thoroughly to mix them. Means are provided to separate the flocculated material from the liquid effluent. Such means to agitate the sewage and the chemical includes an agitating module. The agitating module includes a casing, an intake header at one end of the casing, an outlet baffle chamber at the other end of the casing and a plurality of conduits in the casing communicating between the header and the baffle chamber. Each of the conduits include a plurality of baffles extending from their walls to agitate sewage passing therethrough. A substantially horizontal perforated partition baffle is provided which spans each of the conduits lengthwise and separates each conduit into a normal flow agitating portion above the partition baffle and a reverse flow cleaning portion below the partition baffle. The plurality of baffles are located in the normal flow agitating portion. An effluent-flow-operated flap valve is hinged on the discharge ends of each of the conduits for closing off the normal flow agitating portions when effluent is flowing in a direction opposite to the normal direction of flow, and for opening the normal flow agitating portion when the effluent is flowing in the normal direction of flow.
In U.S. Pat. No. 3,768,648, is disclosed a settler assembly which utilizes a plurality of inclined passages of chevron cross-section configuration. The chevron configuration provides a higher critical flow rate than passages having square, circular, hexagonal and the like cross-sectional configuration.
Swelberg, Canadian Pat. No. 1,074,928 issued Apr. 1, 1980 provides a package water treatment plant for producing drinkagle water from impure water. It incorporates a plurality of coagulation chambers in communication with a primary settling tank which in turn is in communcation with a secondary settling tank. The secondary settling tank incorporates a plurality of inclined coextensively arranged chevron cross-sectional settling tubes which greatly increase the rate of sedimentation of the impure water for subsequent delivery to a third settling tank. The third settling tank incorporates a typical sand filter for removing or catching particles before the water is channeled into a storage tank. The storage tank is arranged to permit a bacteria killing source to be discharged into the stored water.
Another sewage treatment system is the so-called CANWEL system (a trade mark of Central Mortgage and Housing Corporation, Canada). The system consists of an absorption bio-oxidation (A-B) reactor, a sludge separator, a chemical reactor-clarifier, an ozone reactor, and an optional sludge thickener, depending upon the method proposed for sludge disposal.
The raw or comminuted domestic or general municipal (non-industrial) sewage enters the A-B reactor where, in the aeration chamber-air lift, it is contacted with activated sludge and distributed into the top layers of the reactor mixed liquor. The activated sludge consists of a mixed microbial population together with powdered activated carbon.
The sludge is kept in a downward oriented motion, maintained by withdrawing sludge from the bottom of the reactor and recycling it to the top via airlift. The organic matter present in sewage is primarily removed from the processed wastewater by adsorption on biological solids and activated carbon. Then, as the sludge solids move downward by gravity and by the downward flow of the mixed liquor, the adsorbed organics are bio-oxidized. Along the bio-oxidation of organics, the nitrifying bacteria present in mixed liquor suspended solids transform the ammonia to nitrite and nitrate. As a result of these bio-reactions, the concentration of the dissolved oxygen in the reactor liquor drops from about 1.5 mg/1 at the top layers to near zero at the bottom layers. At this low level of dissolved oxygen, nitrite and nitrate replace oxygen as the final electron acceptor in the respiration chain, resulting in microbial denitrification. The mixed liquor then overflows via a submerged pipe into the sludge separator.
The mixed liquor withdrawn from the bottom of the A-B reactor overflows into the sludge separator where the solids are separator from the liquor by settling, and recycled back into the reactor.
The A-B reactor system is consistently "filled" with sludge, the yield, or excess, of which automatically overflows with the treated waste-water from the sludge separator into the chemical reactor-clarifier. Here, by the addition of a chemical coagulant, phosphorus and colloidal substances are precipitated, and then removed together with excess biological solids in a fluidized bed of chemical sludge.
The clarified effluent from the chemical reactor-clarifier is air-lifted into the central column of the ozone reactor where it is contacted with ozone in countercurrent fashion. Oxidation of residual organics then takes place in the main tank. At this point the effluent is suitable for final disposal and may be considered for undiluted surface discharge, discharge to storm systems or small streams and lakes, or for utility use.
The excess chemical sludge together with the excess biological sludge continuously overflows into a sludge thickener, from where the thickened sludge is periodically withdrawn and disposed of by conventional or other means. The supernatant is returned to the A-B reactor.
The CANWEL system also includes a water polishing sub-system which consists of mixed media filtration, optional pH balance control, optional reverse osmosis (R.O.) and disinfection with ozone. The mixed media gravity filtration unit accepts raw water from an approved source and of a quality with potential for municipal and domestic use, and removes a high percentage of suspended solids. Where required, appropriate chemicals are added automatically in controlled amounts to maintain pH balance.
Where required, the R.O. process demineralizes the water by reducing to safe levels the incidence of dissolved salts. The process also deals effectively with colloidal solids (e.g., asbestos), bacteria, and dissolved organic materials. A concentrated brine with other rejects is accumulated for disposal.
The water flows into the ozone contact chamber for oxidation and destruction of any residual viral and bacterial organisms. This polished water is now suitable for all domestic purposes and will meet the requirements for municipal water.
(iii) Deficiencies of the Prior Art
In spite of the above described prior art, there is still a need for an improved sewage treatment system, method and apparatus to provide an effluent that meets the standards for human consumption out of sewage, in the shortest possible time. The effluent should be so pure that there would be no need for the cleaned effluent to pass through conventional first and second filtering tanks.